1. Field of the Invention
The present invention relates to subsurface well equipment and, more particularly, to an eccentric subsurface safety valve having a curved flapper.
2. Description of the Related Art
As is well known, after an oil and gas well is drilled, casing is cemented in place therein, and a string of production tubing, including various downhole tools (the combination of which is generally referred to as a "well completion") is disposed within the casing and used to produce hydrocarbons to the earth's surface. Historically, oil and gas producing companies have been interested in drilling small diameter wells that utilize the largest possible production tubing within the casing. This strategy allows the company to lower drilling costs, by drilling a smaller hole, and maximize profits, by the use of large production diameters. To gain the full potential of a well and maximize serviceability, all internal restrictions within the production tubing must be minimized throughout the completion.
A standard downhole well tool in well completions is a subsurface safety valve, which is commonly used to prevent uncontrolled fluid flow through the well in the event of an emergency, such as to prevent a well blowout. In a typical well completion, the subsurface safety valve is located near the top of the completion. As such, it becomes necessary that the internal diameter of the valve be as large as possible so as to enable passage of various well tools through the valve to other components below the valve. This large internal diameter requirement for the valve coupled with commercial incentives to reduce the inside diameter of the support casing creates a restrictive design envelope for the valve. If the outside diameter of the valve is just below the inside diameter of the casing and a flat flapper is used as the sealing mechanism in the valve, then the inside diameter of the valve is limited by geometric considerations, as is well known in the art. For some completions, the resulting inside diameter is unacceptable. In an effort to increase inside diameters, curved flapper valves were developed. However, the industry continues to move towards the use of tubing and casing having larger and larger diameters. For example, with the advent of directional drilling, the industry has recognized the economic advantages of drilling one relatively large, generally vertical, main well bore, and then directionally drilling and completing multiple lateral wells therefrom. Use of multiple lateral wells stemming from a main well is also environmentally advantageous in that it results in less disturbance to the earth's surface, or a smaller "footprint," as compared to the relatively large disturbance/footprint when drilling and completing numerous individual vertical wells of the traditional type. The movement towards use of these larger diameters has given rise to a need in the industry for a subsurface safety valve wherein the outer diameter of the valve remains constant, or of a heretofore standard dimension, but wherein the inside diameter of the valve is increased.